1. Field of the Invention
This invention relates to a disk reproduction apparatus such as a CD (compact disk) player or and CD-ROM (read only memory) player. The invention also relates to a method of making high speed accesses to a disk reproduction apparatus.
2. Description of the Related Art
Digital record/playback apparatuses are currently winning a larger and larger market share of the acoustic equipment market. This trend is due to the benefits of high density audio recording and of playing back the high density recorded audio data with a good fidelity. This technology is often called a digital audio technology.
The digital acoustic systems are theoretically superior to analog acoustic systems. Audio characteristics are independent of the characteristics of the recording medium. Audio systems employing disks for their recording mediums are called DAD (digital audio disk) systems. Playback systems for playing back audio data from recorded disks (recorded disks are generally employed in DAD systems typically contain an optical playback system, an electrostatic playback system, a mechanical playback system, and the like.
Of optical playback systems, the optical disk player using a CD is the most typical. Geometrically, a CD is 12 cm in diameter and 1.2 mm in thickness. Structurally, a CD is a transparent disk made of synthetic resin and covered with a metal thin film with circularly arrayed pits. These pits represent digital data as obtained by pulse-code-modulating (PCM) the analog acoustic information as picked up. The disk is rotated at a rotating speed in the range from approximately 500 to 200 rpm. A CLV (constant linear velocity) system is used for driving the disk. For playing back acoustic data from the disk, an optical pick-up (involving a semiconductor laser, a photo-electric conversion element, and the like) is linearly moved from the inner peripheral side to the outer peripheral side of the disk. A linear tracking system is used for moving the pick-up. A disk with a 1.6 .mu.m track pitch is capable of storing a great amount of acoustic information. A single side of the disk stores enough acoustic data to provide one hour of continuous stereo sound.
There are two types of data stored on the disk: digital audio data from the digitized audio signal for stereo playback, and subcode data for providing good operability and highly sophisticated functions. The subcode data includes eight types of data denoted as P to W, and has a total of 8 bits each for one frame (588 channel bits).
Of the eight types of subcode P to W, subcode P represents a point where one musical piece is changed to another in the disk. Six subcodes R to W are for displaying an image on a CRT display of a television set. This image can be synchronized with the digital audio data. Subcodes R to W are applied to a decoder, which is specially designed for image display. A still picture is then displayed on the display under control of the decoded data from the decoder.
Subcode Q is also called address data. This data is and stored in a program area radially ranging from 25 to 58 mm on the disk. This address data contains a musical piece number (track number: TNO) of each musical piece stored, a measure number (INDEX), time lapse (TIME) of each musical piece from the instant that its playing begins, and a total time lapse (ATIME) from the starting point of the program area.
The disk further stores TOC (table of contents) data in the read-in area ranging 23 to 25 mm. TOC data represents the start position (address) of each musical piece in terms of a total lapse time.
Thus, a great amount of data is recorded on the disk. The particular disk player used with this disk must search for the intended data from the large amount of data at the highest possible speed. The search function is therefore an essential feature of the disk player. It is subcode Q which is used for executing the search function.
The CD player mentioned above was originally developed to playback musical data. The large memory capacity of the CD, however, provides another use of the CD. This new use of the CD is known as a CD-ROM reproduction apparatus. In this system, the CD is used as a ROM or a read only recording medium. Many types of digital data can be stored in the memory area typically used for storing audio data in conventional CD players. To record and reproduce the digital data, the CD-ROM reproduction apparatus employs a conventional record/playback format. Additionally, a new format is also used. The two formats will be described below.
In CD playback systems, analog audio signals of two channels (L and R) are sampled at 44.1 kHz. The sampled data is digitized and recorded onto the CD as 16-bit digital audio data. In CD-ROM reproduction systems, on the other hand, the 16-bit digital audio data of both channels is divided into two pieces of data, each including 8 bits (one byte data). The digital data is stored as a data unit (one block) of 2352 bytes. One block of data includes:
(1) Sync pattern for checking the start of each block (12 bytes);
(2) Header address as the address data of the block (every 1/75 sec.), which corresponds to subcode Q (4 bytes);
(3) Data for user (2048 bytes=2 K bytes); and
(4) Data for data error detection and correction (mode I) or data for user (mode II) (288 bytes).
Mode I is used when data error cannot be corrected by the parity correction for both channels C1 and C2 in the audio data playback format. If Mode I is used, the error rate can be improved up to 10.sup.-12. This low error rate enables the CD to be used as the recording medium for a computer.
Mode II allows the operator to use the memory area of 288 bytes. When Mode II is used, the error rate is 10.sup.-9 at most, but the amount of usable data in one block is increased to: EQU 2048+288=2336 bytes.
Incidentally, one block corresponds to one frame of subcode. One block of data is reproduced every 1/75 sec. In Mode I, this is 2 K bytes of data per block. The data transfer rate of the CD-ROM reproduction system is EQU 2K.times.75=150 K bytes/sec.
Therefore, in the case of the disk which is used in the CD-ROM reproduction system and has a memory capacity of the data providing one-hour continuous playing on each side, the data recorded therein is EQU 150 K bytes.times.60.times.60=540 M bytes.
The memory capacity of the CD amounts to that of normal floppy disks of 500 to 1000 or 270,000 documents each containing 200 letters written therein. In this respect, the CD-ROM disk is a data recording medium featuring a low error rate and a tremendously large storage capacity. A portion of the CD-ROM disk may be used, if necessary, for digital data storage, while the remaining portion of the disk can be used for digital audio data storage. Thus, the digital data and the audio data may simultaneously be recorded onto the disk by switching the control bit of subcode Q according back and forth between digital data and audio data.
The CD-ROM disk is a data recording medium. To process the data recorded, therefore, a host system or a host computer is required. In use, an operator operates a keyboard of the host computer, to cause the host computer to send a command to the CD-ROM reproduction apparatus. In response to the command, the reproduction apparatus searches for the address of the desired data. Following the search, the reproduction apparatus transfers the data specified by the searched address, back to the host computer.
Because CD-ROM reproduction apparatuses are used as a data memory apparatuses, high speed data access is a desirable performance feature. To make a data access, a, conventional CD disk uses subcode data Q. The CD-ROM disk, on the other hand, uses the header address data. Therefore, if the subcode data Q (in the case of the CD-ROM disk, the header address data) read out is incorrect, the data access cannot be done at a high speed. To solve this problem, the conventional CD player checks the continuity of subcode Q at the present position of the pick-up and subcode data Q read out previously by one frame before. (one block) If the addresses are continuous, the present subcode data Q as read out is deemed correct.
The above method, however, still has problems. In the above method, it is assumed that the pick-up is moved for data access, it is assumed that the pick-up reads out subcode data Q, and it is assumed that the first data Q read out is correct. In the conventional CD-ROM reproduction apparatus, a controller decides whether or not the data first read out data is incorrect or invalid. The time required to make this decision is wasteful and slows the data access.
More specifically, the subduring code data Q first read out following the pick-up movement, is not continuous with the data last read out before the movement. Therefore, the controller decides that the first read out subcode Q is invalid. Then, the controller decides a continuity of the subcode data Q first read out after this movement and the subcode data Q read out after a time elapse amounting to one frame (one block) reproduction from that position. If these data are continuous, the controller decides that the subcode data Q is correct. The time it takes the pick-up to reproduce one frame after the pick-up moves (1175sec.) is wasted time. This time loss hinders the high speed data access operation.